1、Coal-Producing Tectonic Environments This final chapter in the investigation of coal sedimentation is concerned with depositional aspects of the highest order of magnitude, namely, the influence of the crustal setting on peat accumulation. This is a broad and complex field which draws on information
2、, gathered from many different disciplines of the earth sciences. Some of these are currently evolving quite rapidly, while others re in a “mopping up” stage, insensu Kuhn (1970) and Walker (1973), following recent scientific revolutions. An example of the latter is the replacement of the geosynclin
3、al hypothesis in the early 1970s by the concept of plate tectonics. Even after a life span of 20 years, this new paradigm is still in the process of being refined and fitted out with conceptual subsets, as shown by the current emphasis on terrane analysis. It is therefore not possible at this stage
4、to make a definitive statement on the chosen subject, but merely to outline the principle on which a modern geotectonic classification of coalfields can be established. Even this modest goal is fraught with difficulty, because the change from the predominantly static geosynclinal view of global tect
5、onics to its modern, largely mobilistic interpretation has complicated the tectonic classification of some coalfields. While the tectonic status of many coalfields, e.g. those in foredeeps or foreland basins has changed relatively little, the setting of coals found in inter- and intramontane troughs
6、, i.e. within orogenic cordilleras, cannot be properly assessed without very careful study. According to the geosynclinal concept, practically all of these intradeeps, together with fore- and backdeeps, their extra-orogenic counterparts, were regarded as part of a group of molasses basins, the devel
7、opment of which accompanies or follows “terminal geosynclinal tecto-orrgenic” (Aubouin 1965). This fixist and strictly sequential interpretation (highlighted by the term “epieugeosyncline” of Kay 1951) has no place in modern geotectonic analysis, which views most orogenic belts as collages of autoch
8、thonous and allochthonous terranes, i.e. as tectono-stratigraphic assemblages with possibly coeval but heterogeneous stratigraphic records reflecting their origin in different geological and geographical domains (Monger and Price 1979, Monger et al. 1982). The tectonic setting, which influenced the
9、formation of an allochthonous terrane assemblage before accretion, may have been very different in style and physically far removed from its resting place after docking. It follows that a multi-terrane orogen may contain a variety of coals formed at different times before and after terrane accretion
10、. Moreover, contemporaneous pre-accretionary coal deposits formed in different terranes are likely to vary in coal types, coalification histories and tectonic styles, and all of these will in turn differ from the post-accretionary molasses coals, which alone reflect the conditiona prevailing in the
11、orogen itself. Indeed, the situation may even be more complex, as will be discussed in Chap.9.3.2.2. Plate tectonics has created its own nomenclature, of which only the essential terms will be used here. They will be supplemented by terms which are either descriptive, and therefore independent of ge
12、otectonic theory, or which have stood the test of time because they are useful in spite of their generic association with now obsolete concepts. For example, the expressions “mio-” and “eugeosynclinal assemblage” have been kept here as reference term for shallow water marine (mainly shelf), and ocea
13、n floor pelagite, turbidite and ophiolite associationa, respectively. Moreover, reduced to a “miogeocline”, the miogeosyncline has in the North-American literature become a standard term for autochthonous, sedimentary terrace wedges onlapping continental margins. Also tectonic attributes of sediment
14、s, such as “synorogenic” flysch and “late syn- (folded) to postorogenic (non-folded)” molasses, respectively, can still be used in a plate tectonic context without unduly corrupting their relatively loose definitions. Particularly in the discussion of coalfields situated near convergent plate edges,
15、 the concept of molasses as the product of the destruction of the uplifted orogen is very useful. As in the previous discussion, it is not the purpose of this chapter to give detailed descriptions of a large number of cases but to select a few typical examples of coalfields and relate the essence of
16、 their architecture to their respective plate tectonic settings. 1 Early Examples of a Tectonic Classification of Coalfields Large-scale coal formation can take place only in actively subsiding regions, for example in sedimentary basins. It is possible therefore to characterise the geotectonic envir
17、onment occupied by a coal measure sequence in a manner similar to that which is applied to other sedimentary environments. Stutzer (1920) and Stille (1926) were among the first to recognise the genetic links between tectonism and the formation of coal. Stille, in particular, referred to the striking
18、 difference in terms of basin fill, number of coal seams present, their average thickness and proportion in relation to total coal measure thickness, which exist between the Carboniferous and Tertiary coal measures of Europe. He attributed such dissimilarities to contrasting degrees of crustal mobil
19、ity in the areas affected by the two main European coal-forming periods. His results are summarized in Table 9. 1. Even if differences in compaction ratios between the Tertiary brown and Carboniferous bituminous coals are taken into account (to a lesser degree the compaction applies to inter-seam sediments) the contrast is
